4Integrating Peripherals in Embedded Systems cont.

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4-Integrating Peripherals in Embedded Systems
(cont.)
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Analog-to-digital converters

• Why Analog?
• Not everything is digital!
• Even digital signaling has analog aspects (!?)
• Analog circuits and analysis are still necessary
• Physical phenomena (ie the real world) are
  usually analog
• Many sensors are analog (potentiometer,
  phototransistor, thermo-sensor, microphone)
• Many actuators are analog (solenoid, speakers)
• Some signals need to be processed in analog
  domain before conversion to digital
  (amplification, filtering, linearization)

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Analog-to-digital converters
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Digital and Analog Conversion

• A/D transfer function
• 10-bit ADC converter
• 1024 voltage levels between 0V and VREF
• 10-bit digital value
• Usually VDDVREF
• How does D/A and A/D conversion work?

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D/A Conversion

• D/A is simpler than A/D
• Different resisters and an inverted OpAmp, to
  implement a weighted summer.
• Example 4-bit D/A
• If D3D2D1D00001 (i.e., 0V,0V,0V,-5V)
• Vo-1.1(D0/17.6D1/8.8D2/4.4D2/2.2)
• 0.3125V
• If D3D2D1D01111 (i.e., -5V,-5V,-5V,-5V)
• Vo-1.1(D0/17.6D1/8.8D2/4.4D2/2.2)
• 4.6875V

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A/D Conversion

• Use D/A converter to generate different analog
  values and compare
• Control logic decides which values to try
• When comparison complete, best match is put on
  output
• How can D/A be matched to input in fewest steps?

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Successive Approximation

• Matching strategies
• Counting conversion (slow)
• Successive approximation (faster)
• Successive Approximation
• Basically binary search
• 10 steps instead of 1024

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Digital-to-analog conversion using successive
approximation
Given an analog input signal whose voltage should
range from 0 to 15 volts, and an 8-bit digital
encoding, calculate the correct encoding for 5
volts. Then trace the successive-approximation
approach to find the correct encoding. 5/15
d/(28 - 1)d/255 d 85
Encoding 01010101
Successive-approximation method
½(Vmax Vmin) 7.5 volts Vmax 7.5 volts.
½(5.63 4.69) 5.16 volts Vmax 5.16 volts.
½(7.5 0) 3.75 volts Vmin 3.75 volts.
½(5.16 4.69) 4.93 volts Vmin 4.93 volts.
½(7.5 3.75) 5.63 volts Vmax 5.63 volts
½(5.16 4.93) 5.05 volts Vmax 5.05 volts.
½(5.63 3.75) 4.69 volts Vmin 4.69 volts.
½(5.05 4.93) 4.99 volts
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Analog to Digital Recording Chain
ADC
Microphone converts acoustic to electrical
energy. Its a transducer.
Continuously varying electrical energy is an
analog of the sound pressure wave.
ADC (Analog to Digital Converter) converts analog
to digital electrical signal.
Digital signal transmits binary numbers.
DAC (Digital to Analog Converter) converts
digital signal in computer to analog for your
headphones.
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Analog Representations of Sound
Magnified phonograph grooves, viewed from above
When viewed from the side, channel 1 goes up and
down, and channel 2 goes side to side.
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Analog to Digital Conversion
Instantaneous amplitudes of continuous analog
signal, measured at equally spaced points in time.
A series of snapshots
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Analog to Digital Overview
Sampling Rate
How often analog signal is measured
samples per second, Hz
Example 44,100 Hz
Sampling Resolution
a.k.a. sample word length, bit
depthPrecision of numbers used for
measurement the more bits, the higher the
resolution.
Example 16 bit
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Sampling Rate
Determines the highest frequency that you can
represent with a digital signal.
Nyquist Theorem
Sampling rate must be at least twice as high as
the highest frequency you want to represent.
Capturing just the crest and trough of a sine
wave will represent the wave exactly.
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Common Sampling Rates
Which rates can represent the range of
frequencies audible by (fresh) ears?
Most software can handle all these rates.
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3-bit Quantization
A 3-bit binary (base 2) number has 23 8 values.
Amplitude
Time measure amp. at each tick of sample clock
A rough approximation
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Common Sampling Resolutions
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The Digital Audio Stream
Its just a series of sample numbers, to be
interpreted as instantaneous amplitudes one for
every tick of the sample clock.
Previous example
11 13 15 13 10 9 6 1 4 9 15 11 13 9
This is what appears in a sound file, along with
a header that indicates the sampling rate, bit
depth and other things.
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Audio File Size
CD characteristics
- Sampling rate 44,100 samples per second (44.1
kHz)
- Sample word length 16 bits (i.e., 2 bytes)
per sample
- Number of channels 2 (stereo)
How big is a 5-minute CD-quality sound file?
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Audio File Size
How big is a 5-minute CD-quality sound file?
44,100 samples 2 bytes per sample 2
channels 176,400 bytes per second
5 minutes 60 seconds per minute 300 seconds
300 seconds 176,400 bytes per second
52,920,000 bytes c. 50.5 megabytes (MB)
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MPEG Compression

• MPEG 1-Layer 3 (.mp3)
• Motion Picture Experts Group
• Different levels for different purposes
• E.g. MPEG 2 used for DVDs
• Takes out parts of the sound signal that humans
  cant hear
• How does the size change?
• Lossy compression

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MPEG 3 Audio Compression (mp3)

http//www.iis.fraunhofer.de/amm/techinf/layer3/
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MPEG Compression